Introduce cross-db ILIKE

Instead of having to have explicitly handle each different database every time we want to do a case insensitive like (such as [here ](https://github.com/owncloud/core/blob/3e3d460729c515e5723587abc30b4d7955173d12/lib/private/files/cache/cache.php#L469)) it would be a lot better to have a cross db way to handle this.

This introduces handling of `ILIKE` inside the database adapters to make sql statements using `ILIKE` work across database backends.

cc @DeepDiver1975 @PVince81